Dynamic Hazards

The other type of hazard associated with the combinational networks is called as a dynamic hazard.

It also has the same effect as that of static hazards i.e. false output.

Dynamic hazards occur when the output of the circuit is to change from one logic level to the other one but a momentary false output signal occurs during the transient behaviour.

The dynamic hazards are illustrated in Figure below.

Definition of Dynamic Hazard :

We can define the dynamic hazard as a transient change which occurs three or more times at the output of a logic circuit, when the output is supposed to change only once during the transition between two inputs states which differ in the value of one variable.
Illustration of Dynamic Hazards :

Consider the network shown in Figure below. It can be proved that this network is free from static 0 as well as static 1 hazards.
Steady state behaviour :

For understanding the transient behaviour, assume that the gates G3 and G5 do not have any propagation delays.

The propagation delays of the other gates are as follows :
(1) G1 switches faster than G2.
(2) G2 can switch faster than G4.

That means G1, G2 and G4 have finite propagation delays. The delay of G4 is longest and that of G1 is shortest.
When A changes from 0 to 1 :

When A changes from 0 to 1 the events taking place are as follows :
1. This change propagates through G1 before G2.
2. The input to G3 are simultaneously 1.
3. The output Y changes from 0 to 1.

This is shown in Figure below.

Then when change in A propagates through gate G2, the lower input to G3 becomes zero and the network output again returns back to 0.

Finally when A = 1, the signal propagates through gate G4 to make the lower input to the gate G5 equal to 1 and the Y output again becomes 1.

Thus during the transition of input variable A, the output Y changes 0 ï‚® 1 ï‚® 0 ï‚® 1 and the dynamic hazard is said to have occured.

The detection of dynamic hazards is more difficult than that of the static hazards.

They produce false outputs. Their severity depends on the devices or systems receiving them.

The false outputs in the combinational logic of asynchronous sequential networks can cause malfunctions.

So combinational logic circuits which are specially designed for static hazard free operation can also be free of dynamic hazards.

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